Envelope tracking power amplifier systems are known in the art, and generally comprise the provision of an input signal to be amplified on an RF input path to a signal input of a power amplifier, and an envelope path for generating a modulated power supply based on the input signal, with the modulated power supply being provided to a power supply input of the power amplifier.
An exemplary prior art envelope tracking power amplifier system is illustrated in FIG. 1(a). A signal to be amplified, in this example being a complex signal having I and Q components, is generated by a source block 108. The signal to be amplified is provided to a delay block 106 which provides a delay alignment. The output of the delay block 106 provides a signal to the RF input path and a signal to the envelope path. The signal in the RF input path is provided to an RF up-conversion block 104 which converts the input signal to an RF signal, and then provides the input to a power amplifier 102. The envelope path receives the signal from the delay block 106 in an envelope detector block 114, which detects the envelope of this signal. The thus detected envelope is provided to an envelope shaping block 112 which shapes the envelope, and the output of the envelope shaping block 112 is delivered to a supply modulator 110. The output of the supply modulator 110 provides the modulated supply voltage to a supply terminal of the power amplifier 102. The output of the power amplifier 102 provides an RF output signal.
The delay block 106 provides delay adjustment to maintain precise timing alignment between the RF signal in the input path applied to the input of the power amplifier 102 and the modulated power supply voltage applied to the supply terminal of the power amplifier 102.
The envelope shaping block 112 comprises a shaping table and implements an envelope shaping function, and may be provided in order to remove signal troughs from the signal in the envelope path.
RF linearisation of the circuit of FIG. 1(a) may be achieved by adjusting the envelope shaping function of the envelope shaping block 112.
FIG. 1(b) represents an alternative implementation of a prior art envelope tracking power amplifier system. The system of FIG. 1(b) corresponds to the system of FIG. 1(a), and like reference numerals are used to illustrate similar elements. FIG. 1(b) but additionally includes a DPD (digital pre-distortion) block 116 positioned between the output signal of the delay block 106 and the input of the RF up-conversion block 104. The DPD block 116 provides pre-distortion of the RF waveform and acts in addition to or alternatively to the envelope shaping function 112, to achieve RF linearization of the system. In this implementation the pre-distortion of the complex signal occurs after the point at which the envelope is detected.
FIG. 1(c) illustrates a further modification to the arrangement of FIG. 1(a) for a prior art envelope tracking power amplifier system. The system of FIG. 1(c) corresponds to the system of FIG. 1(a), and like reference numerals are used to illustrate similar elements. In FIG. 1(c) a DPD block 118 is connected between the output of the source block 108 and the input to the delay block 106. Thus pre-distortion is applied before the point at which the envelope is detected. This pre-distortion function provided by the DPD block 118, either alternatively to or additional to the envelope shaping block 112, achieves RF linearization of the system.
Prior art envelope tracking systems maintain a 1:1 relationship between the instantaneous supply voltage applied to the power amplifier and the instantaneous envelope of the RF input signal applied to the power amplifier. That is, for each value of the envelope of the RF input there is one corresponding value of supply voltage. As a result of this 1:1 relationship, a DPD block needs only to be one-dimensional and receive only a signal representing the signal input to the power amplifier. The operation of the non-linear envelope detection operation provided by the envelope detector block 114 increases the bandwidth of the signal in the envelope path. In a typical implementation the supply modulator bandwidth, i.e. the bandwidth in the envelope path, is chosen to be 1.5 to 3 times the bandwidth of the RF path.
The maximum bandwidth an envelope tracking system can support is typically determined by the design of the supply modulator 110. In particular the maximum current and voltage signal slew rate the supply modulator can support without introducing significant distortion typically determines the maximum bandwidth of the envelope tracking system.
The maximum RF bandwidth required for the RF input path of most current cellular systems is 20 MHz, but it is expected that this will rise to 40 MHz or higher in future systems.
Future WiFi systems such as 802.11ac will be required to support an RF (radio frequency) bandwidth in the input path of up to 160 MHz.
The design of a supply modulator having a bandwidth of greater than 40 MHz is extremely challenging using existing semiconductor technology. Furthermore, the sub-nanosecond timing alignment accuracy which is required to be controlled by the delay block 106 for these higher bandwidth systems becomes increasingly difficult to achieve and maintain.
The bandwidth of the supply modulator and the RF to envelope delay matching requirements can be significantly reduced by using partial envelope tracking. This requires a reduced bandwidth supply reference signal to be generated in the envelope path, subject to the constraint that the instantaneous voltage of the supply reference signal must be greater than the instantaneous voltage of the full bandwidth RF envelope signal at all times in order to provide sufficient supply voltage to amplify the input signal in the power amplifier 102.
This can be illustrated with reference to FIG. 2.
FIG. 2 illustrates a plot of voltage supply against time for an envelope signal having a reduced bandwidth and for an envelope signal having a full (normal) bandwidth. Reference numeral 202 identifies the supply voltage for a reduced bandwidth envelope signal, and reference numeral 204 illustrates the supply voltage for a full bandwidth envelope signal.
Without the constraint that the voltage of the supply reference signal must be greater than the voltage of the full bandwidth RF envelope at all times, the supply voltage may in fact be lower than the amplitude of the RF envelope at certain times. This constraint is not met if a simple low pass filter is introduced in the envelope path. It then is not possible to linearise the RF signal using RF pre-distortion due to excessive power amplifier compression.
With such a system, applying this constraint, there is no longer a 1:1 mapping between the instantaneous RF envelope signal and the instantaneous supply voltage, so it is not possible to correct system non-linearity using simple one dimensional DPD blocks as shown in FIGS. 1(b) and (c) (element 116 of FIG. 1(b) and element 118 of FIG. 1(c)).
In the prior art there are disclosed methods for generating reduced bandwidth envelopes. Reference can be made to IEEE IMS2009 paper J. Jeong et al. “Wideband Envelope Tracking Power Amplifier with Reduced Bandwidth Power Supply Waveform” and A. Cesari et al. “A DSP Structure authorising Reduced-Bandwidth DC/DC converters for Dynamic Supply of RF Amplifiers in Wideband Applications” Proc. Industrial Electronics Conf. November 2006.
It is an aim of the present invention to provide an improved envelope tracking power amplifier system in which the bandwidth in the envelope path is reduced.